I'm sure the list of high level mob rats that we don't know about would surprise us and that there were a lot more than revealed, but 50% is way too high. Also, considering the era they were alive in, we can't dismiss Frank Costello and Carlo Gambino outright as rats just because they were successful. Law enforcement was all over them in the later part of their careers and Costello even had to do some prison time. Now they may have ratted rivals out in their path to the top before they became a major law enforcement target like we know Luciano did, but there is no evidence.